The gas turbine engine air must be compressed before it's use in the combustor. The compressor may be either axial, centrifugal, or a combination of the two.
In certain engines axial stages are followed by a centrifugal final stage. Such an arrangement is shown in U.S. Pat. No. 5,165,226 along with delivery of the compressed air to the combustor.
Air leaving the centrifugal stage is at a high velocity traveling with a major tangential velocity component. This velocity passing into the annulus surrounding the centrifugal stage must be transformed to pressure energy, and the air delivered to the combustor with minimum pressure loss. The intense swirl must also be reduced. The swirl continues abated somewhat into the combustor, passing through the side wall cooling openings. Some swirl facilitates mixing, while a high swirl creates ignition stability problems in the combustor.
It's desirable that a aircraft gas turbine engine be as small as possible both as to length and diameter. Therefore the conversion of velocity energy to pressure energy must be accomplished in a minimum volume.
Conventional practice uses a bent trumpet diffuser to transform this high velocity air. The trumpet receives air tangentially from the compressor discharge annulus and expands the flow area. This reduces the velocity and velocity head of the airflow. The trumpet then bends toward the combustion supply volume while still expanding the flow area.
The reduction of velocity before the turn is used to reduce the loss during the turn. Secondary flow in the form of twin circular paths occurs during the turn, resulting in some increased pressure drop in the remainder of the trumpet. Attempts to minimize this loss puts further constraints on the freedom to further shape the trumpet. The additional increased flow area after the bend is an attempt to reduce the exit loss.
Straightening vanes have been placed near the exit of the trumpet to reduce the swirl in the chamber. These have been only partially successful and also introduce further pressure loss.